With the results as given, a product that returns an integer as the result of (1 div 1), or of (2.0 div 2.0), fails the test.

The language rules for such cases say that the result must be an instance of xs:decimal; they do not say that it cannot also be an instance of xs:integer. The general rule is that where a function is required to return a value of type T, is is permissible to return a value annotated with a subtype of T if the value is in the value space of the subtype. The test should be changed so that where the result is an xs:decimal and is a whole number, returning an instance of xs:integer is acceptable.